Natural gas is a relatively low density material composed primarily of alkane compounds, which are alternately termed paraffins. Large deposits of natural gas are found in numerous regions of the world, many of which are relatively unpopulated and lack significant markets for natural gas. It is possible to transport the natural gas to more highly populated regions where there is greater market demand for the natural gas. However, there are a number of practical and economic considerations which limit the feasibility of transporting natural gas long distances. Foremost is the fact that many remote regions, which possess large natural gas deposits, lack gas pipeline infrastructures for transporting natural gas to more highly populated regions. Even where gas pipeline infrastructures are present, pipeline transport of natural gas is often prohibitively expensive for long distances due to the low density of natural gas.
Natural gas can alternatively be transported as compressed gas in large vessels aboard transport vehicles, such as trucks, rail cars, or ships. However, transport of compressed natural gas in accordance with this alternative is likewise often prohibitively expensive for long distances. Natural gas can be more economically transported over long distances if the natural gas is first cryogenically processed to liquified natural gas (LNG). However, this alternative is not entirely satisfactory because the high-pressure cryogenic liquifaction process is expensive and LNG transport is often impractical because there are only a limited number of facilities which are equipped to ship or receive high-pressure and low-temperature LNG. As such, a need continues to exist for a practical and economically feasible means of transporting natural gas over long distances from remote regions to regions where there is substantial market demand.
One approach which has long been considered as a potential solution to the high cost of transporting natural gas over long distances is to first chemically convert the natural gas to heavier liquid hydrocarbon products having the same or greater utility than the natural gas from which the products are derived. The resulting liquid hydrocarbon products can be economically transported using the same established infrastructure, which is used to transport conventional liquid hydrocarbons such as crude oil, gasoline, jet fuel and the like.
A class of liquid hydrocarbons termed oxygenates, which includes alcohols and ethers, such as methanol, ethanol, dimethyl ether, and the like, has broad utility as chemical feedstocks, solvents, propellants, and fuels. Oxygenates are particularly desirable as fuels for reciprocating engines and gas turbines in transportation and stationary power generation applications because oxygenates have clean burning characteristics (i.e., oxygenates generate low-emission exhaust when burned). Furthermore, oxygenates can be transported at relatively low cost.
Chemical conversion processes are well known for converting natural gas to oxygenates. For example, various natural gas reforming processes are used in combination with catalytic synthesis processes, such as the Lurgi low-pressure methanol process, to produce oxygenates, such as methanol, from natural gas. Presently utilized reforming processes are relatively inefficient, producing large amounts of waste heat and unwanted byproducts such as CO2. In addition, the initial capital investment required for such processes is extremely high. Present reforming and methanol synthesis processes operate at high temperatures and pressures also result in high production costs. Accordingly, the substantial cost of methanol obtained from such conventional production technology generally restricts methanol to higher value uses than fuels. The primary applications for synthesized methanol are chemical feedstocks and solvents, although synthesized methanol does have some limited applications as an additive to conventional crude oil-derived fuels and as a neat fuel in a few demonstration or niche-market applications.
It is apparent that the above-recited need for a practical and economically feasible means of transporting natural gas over long distances can be solved by a low-cost process for converting alkanes, such as methane, ethane, and the like, found in natural gas to the corresponding alcohols and ethers, such as methanol, ethanol, and the like. Not only are alcohols and ethers valuable products, but as liquids they have a relatively high density which renders them more economically transportable than natural gas over long distances. Accordingly, the present invention recognizes a need for a low-cost process for converting alkanes to oxygenates.
It is generally an object of the present invention to provide a process for converting alkanes to oxygenates. More particularly, it is an object of the present invention to provide an alkane to oxygenate conversion process which has relatively low initial capital equipment costs and which has relatively low operating costs. These objects and others are accomplished in accordance with the invention described hereafter.